Hundreds of millions of aluminum die-cast parts are made in the United States every year. These parts are produced by many manufacturers on a number of different types of casting machines, but quality and yield of the die-cast parts are concerns common to all manufacturers. For those areas of the market where quality is a primary concern, particularly for suppliers to the automotive industry, new improved die-casting processes need to be improved to the extent that die-cast pieces can replace aluminum forgings.
Molten metal transfer for aluminum die casting and other processes generally includes two operations: transfer of the molten metal from the melting furnace to the holding furnace, and transfer from the holding furnace to the shot chamber of the die-casting machine. Two important facts in the final quality of the die-cast pieces are the cleanliness of the aluminum transferred into the die and the amount of entrapped air. The processes used to transfer metal between work stations and to introduce the metal into the die strongly affect these two factors. Prolonged contact of the molten aluminum with oxygen will generate oxides which float on top of the aluminum melt, and poor metal handling can cause the presence of these oxides in finished pieces. Poor metal handling can also cause gases to become mixed with the aluminum during freezing in the die to make a solid part, which results in internal voids and weakened parts.
Usually, a single large-volume melting furnace is used to supply molten metal to smaller holding furnaces at die casting machines. The transfer mechanism must be able to provide a continuous supply of molten metal to the holding furnaces. This transfer is usually accomplished by manual or mechanized ladling. Some problems associated with transfer ladles are the buildup of dross and agitation, leading to non-metallic inclusions in the product, and the labor intensive nature of such transfer. Electromagnetic pumps have been used to some extent but those available have proven to be rather fragile and somewhat costly to maintain.
U.S. Pat. No. 4,212,592 (Olich et al.), teaches a self-priming, low friction, electromagnetic, partial-immersion pump for molten metals. There, a rotating magnetic field, perpendicular to molten aluminum flow, generates currents in the molten metal and with an inlet swirl inducer causes rotation of the molten metal about the axis of the pump structure, which comprises a high permeability stainless steel cylinder covered with ceramic mat and silicon carbide. All the core and windings operate outside the molten metal bath, while part of the swirl inducer is immersed. The currents produced, interact with the magnetic field and a torpedo inductor to provide a forward propelling force in the molten metal. A single set of polyphased, solid, small winding wires, and an iron core, are energized to commence pumping action of the traveling magnetic wave type created by multiple windings and balanced polyphase excitation. Air is circulated through the entire pump and around the metal windings and core as a coolant, to keep the environment near the windings and core below their Curie Temperature, in order to maintain the magnetic properties of the core.
In U.S. Pat. No. 4,842,170 (Del Vecchio et al.), monophase electromagnetic field operations are detailed for flow control devices used to control molten metal flow. Here, a variety of torpedo vane styles are utilized. U.S. Pat. No. 4,786,237 (Hans), teaches an induction, complete-immersing pump, utilizing electromagnetic fields to pump liquid aluminum. Inert gas, preferably nitrogen, fed down the length of the pump through a gas feed line, is used to cool the metal and present copper oxidation. All the windings and the core within the housing are immersed in the molten metal bath, and liquid aluminum is drawn through a porous ceramic plate by the electromagnetic forces. The use of small wire induction coils would be difficult to cool and indicates use of polyphase excitation. Here, a major amount of magnetic material must be cooled below its Curie Temperature, here, by the expensive means of an oxygen flushing, metal cooling inert gas.
U.S. Pat. No. 4,988,267 (Yamada), teaches an exterior, electromagnetic pump for supplying molten aluminum to a casting machine. The pump is completely exterior to the molten aluminum bath. The machine has a cylindrical core of ferromagnetic material surrounded by a supported ceramic core protection tube to prevent core erosion by the molten metal. A traveling magnetic field is utilized here, which propels the core in the axial direction against the end of the ceramic core protection tube, and could lead to cracking of the ceramic if clearances are too large.
Use of magnetic materials in the Olich et al., Hans, and Yamada pumps, limits the use of those devices to applications where temperatures do not exceed the Curie Temperature of the magnetic material, or else cooling of the magnetic materials must be provided, and where the saturation flux-density must be kept at a high level.
In U.S. Pat. No. 4,928,933 (Motomura), a molten metal supply system for an injection mold is taught, where an electromagnetic pump has two, separately controlled, sets of coils, that is, immersed coils for heating the molten metal and non-immersed coils for pumping the molten metal. A polyphase power supply is used. Because of the polyphase nature of the groups of windings, insulation requirements of the small wires will limit machine operation to much lower temperatures than the molten material being pumped, and so, significant cooling would be required. U.S. Pat. No. 4,635,705 (Kuznetsov) is another example of pumping molten metal using a polyphase electromagnet pump. Here, a strip casting operation is taught, and a detailed description of traveling waves associated with such polyphase electromagnetic pumps is presented.
None of these pumps provide a simple, co pact self-priming device, where the immersed stage can operate without cooling. It is one of the main objects of this invention to provide a simple, compact, dual stage, self-priming electromagnetic pump, where the immersible stage utilizes only standing magnetic waves and can be operated without cooling if required.